1. Field of the Invention
The present invention relates to a motor control apparatus equipped with a delta-sigma modulation AD converter, and more specifically to a motor control apparatus equipped with a delta-sigma modulation AD converter that is used to detect the current flowing in each winding of a motor.
2. Description of the Related Art
A motor control apparatus for driving motors used in a machine tool, a forging press, an injection molding machine, an industrial robot, or the like, commands motor speed, torque, or rotor position in order to control the operation of each of the motors provided one for each drive axis. In such a motor control apparatus, it is important to accurately detect the current flowing in each winding of the motor. The current value detected on each winding of the motor is converted by an AD (analog-digital) converter into digital data which is used to control the driving of the motor. Successive approximation and delta-sigma modulation are two major types of AD converters used in conventional motor control apparatus, but, the delta-sigma modulation-type is becoming none predominant.
FIG. 8 is a block diagram showing a conventional motor control apparatus that uses a delta-sigma modulation AD converter. Generally, the motor control apparatus 101 which drives and controls a three-phase AC motor 2 includes a power conversion unit 51 which supplies drive power to the motor 2, a current detection unit 52 which detects the value of the current flowing from the power conversion unit 51 to each winding of the motor 2, a delta-sigma modulation AD (analog-digital) converter 53 which converts the value of the current detected by the current detection unit 52 into digital data, and a command generating unit 54 which generates, using the digital data supplied from the delta-sigma modulation AD converter 53, a drive command for commanding the power conversion unit 51 to output the drive power for driving the motor 2. The power conversion unit 51 is, for example, an inverter circuit and/or a converter circuit. The current detection unit 52 actually includes two current detection units one for each of two of the three phase windings of the three-phase AC motor. In the thus configured motor control apparatus 101, the command generating unit 54 generates the drive command based on the digital data acquired by AD-converting the current flowing in each winding of the motor 2.
FIG. 9 is a block diagram showing a conventional delta-sigma modulation AD converter. The delta-sigma modulation AD converter 53 includes two major sections, i.e., a modulator (delta-sigma modulation circuit) 61 and a digital low-pass filter 62, both of which operate on a system clock called a modulation clock whose frequency is generally about several to several tens of megahertz. The modulator 61 converts the input analog data into a high-speed low-bit bitstream signal. A large amount of quantization noise generated here is removed by the digital low-pass filter 62, and the resulting data is output as the digital data. Generally, the digital data from the delta-sigma modulation AD converter 53 is output at a rate reduced by decimating the modulation clock by a predetermined factor. This factor is generally referred to as the decimation ratio.
In the motor control apparatus 101, the rate reduced by decimating the modulation clock by the predetermined factor, i.e., the rate at which the digital data is output from the delta-sigma modulation AD converter 53, is not always synchronized to the digital data acquisition rate (generally, several to several tens of kilohertz), which defines the control period of the command generating unit 54 in the motor control apparatus 101. As a result, the delta-sigma modulation AD converter 53 may not output the digital data when the command generating unit 54 desires to acquire the digital data and, consequently, the command generating unit 54 may not be able to acquire the digital data with timing appropriate for current control. To address this, use may be made of a digital low-pass filter configured to be able to continuously output the digital data for each modulation clock period. With this configuration, the digital data output by the most recent modulation clock can be acquired by the command generating unit 54 at the desired timing for each control period, without having to consider the synchronization with the modulation clock.
Generally, in the motor control apparatus, it is important to clearly identify the time instant to which belongs the value of the current detected by the current detection unit 52 and AD-converted into the digital data to be used for the creation of the drive command in the command generating unit. FIG. 10 is a basic principle diagram for explaining AD conversion and digital data acquisition timings in the motor control apparatus equipped with the conventional delta-sigma modulation AD converter. In principle, the delta-sigma modulation AD converter 53 AD-converts analog data in a given “time interval”, and this “time interval” is defined as a value equal to the modulation clock period multiplied by the decimation ratio. As a result, in the case of the delta-sigma modulation AD converter 53, unlike, for example, the case of an successive approximation AD converter, it is difficult to identify the time instant to which the continuous analog data sampled and AD-converted into the digital data belongs. Therefore, in the case of the delta-sigma modulation AD converter 53, it is common to regard the midpoint of the “digital data output time interval B” as being the “AD conversion time instant”, as illustrated in FIG. 10. More specifically, the command generating unit 54 connected to the delta-sigma modulation AD converter 53 acquires the digital data from the delta-sigma modulation AD converter 53 at time instant A when a time D equal to one half of the “digital data output time interval B” has elapsed from time instant C corresponding to the midpoint of the “digital data output time interval B”. Since the “digital data output time interval B” is defined as a value equal to the modulation clock period multiplied by the decimation ratio, as described above, the time D is specified to be one half of the time interval B. That is, once the “time instant C at which to AD-convert the current value (analog data)” is determined, the command generating unit 54 acquires the digital data from the delta-sigma modulation AD converter 53 when the “time D equal to one half of the modulation clock period multiplied by the decimation ratio” has elapsed from the time instant C. Whether the time D has elapsed or not is determined by the command generating unit 54, but the modulation clock as the system clock of the delta-sigma modulation AD converter 53 is not always synchronized to the system clock of the command generating unit 54, and their clock periods vary due to various factors. In the prior art, therefore, the command generating unit 54 has estimated the time D by assuming ideal conditions in which the modulation clock does not vary. Accordingly, in the motor control apparatus 101 equipped with the delta-sigma modulation AD converter 53, as the modulation clock period varies, the motor control accuracy correspondingly degrades.
For a motor control apparatus that controls a motor by using a delta-sigma modulation AD converter for converting the current value detected on each winding of the motor into digital data, several proposals have been made in the prior art to enhance the current detection accuracy. For example, according to the invention disclosed in Japanese Unexamined Patent Publication No. 2008-147809, the current detection accuracy is enhanced by additionally providing a PLL circuit in order to enhance the accuracy of the modulation clock in the modulator section provided in the first stage of the delta-sigma modulation AD converter.
However, in reality, the modulation period varies due to various factors. In the delta-sigma modulation AD converter 53, when the modulation clock period used as the system clock varies, the length of the “digital data output time interval B” also varies greatly. FIG. 11 is a diagram for explaining variations in the AD conversion and digital data acquisition timings in the motor control apparatus equipped with the conventional delta-sigma modulation AD converter. In FIG. 11, “a” indicates the case where the length of the “digital data output time interval” becomes shorter than the specified time length because the modulation clock period is short, and “b” indicates the case where the length of the “digital data output time interval” is the same as the specified time length because there is no variation in the modulation clock, while “c” indicates the case where the length of the “digital data output time interval” becomes longer than the specified time length because the modulation clock period is long. When the modulation clock period varies, and the length of the “digital data output time interval” varies correspondingly, as indicated by B1, B2, and B3, respectively, the time instant corresponding to the “midpoint of the digital data output time interval B” changes as indicated by C1, C2, and C3. When there is no variation in the modulation clock (the case “b”), the “time instant at which the command generating unit 54 acquires the digital data from the delta-sigma modulation AD converter” coincides with the “midpoint C2 of the digital data output time interval B”, but when the modulation clock period is short (the case “a”), or when the modulation clock period is long (the case “c”), the “time instant C at which to AD-convert the current value (analog data)” which is determined by the “time instant at which the command generating unit 54 acquires the digital data from the delta-sigma modulation AD converter” becomes displaced from the “midpoint of the digital data output time interval B”. In this way, according to the prior art method, when the modulation clock period varies, the motor cannot be driven and controlled with good accuracy.
Further, the command generating unit 54 is operating asynchronously with respect to the modulation clock. Therefore, when such a displacement occurs, there is no knowing whether the digital data obtained by AD conversion is the data obtained by AD-converting the current value at the specified time instant, and it becomes difficult to achieve high-accuracy motor drive control.
Further, when the power conversion unit 51 is a power converter such as an inverter circuit that uses a semiconductor switching device, it is desirable that the AD conversion timing be strictly specified in order to avoid the effects of noise due to the switching, etc., of the semiconductor switching device, but when the modulation clock period varies, as described above, since the length of the “digital data output time interval B” also varies, it is difficult to reduce the effects of noise due to the switching, etc., of the semiconductor switching device.
Furthermore, according to the invention disclosed in Japanese Unexamined Patent Publication No. 2008-147809, since the modulation clock is also transmitted via an “insulating means” to the digital low-pass filter at the subsequent stage in the delta-sigma modulation AD converter, variation in the modulation clock affects the digital low-pass filtering operation, and it is not possible to sufficiently enhance the motor control accuracy.